1) Field Of The Invention
The invention relates to a process for the surface treatment of a sheet-like structure, in particular in the form of web-shaped or sheet-shaped substrates, by means of electric corona discharge. The electric corona discharge takes place in an electrode system between electrode and counter electrode, between which the sheet-like structure is located. The invention also relates to an apparatus for carrying out this process, as well as to sheet-like structures which are treated according to the process or with this apparatus.
2) Prior Art
In many cases, the normally smooth surface of sheet-like plastics structures, in particular of films, presents difficulties to the extent that the film webs, having extremely good slip, readily tend to telescope during winding onto a reel. Further difficulties arise during upgrading to increase the adhesion of printing inks, lacquers, adhesives, vapor-deposited metals and the like on account of the smooth surfaces of sheet-like plastics structures.
Due to their chemical composition, many materials are not conductive to electric current or only to an extent which cannot be measured in practice, and therefore are generally considered as insulators. The low conductivity of these materials is, however, the cause for these materials to often collect on their surface very high electrostatic charges which, in particular in the case of sheet-like polymer products such as films or sheets, can result in a great many disadvantageous disturbances, such as uselessness of the product, and which for this reason are highly undesirable. The charges arise during production, further processing, or use of the films or sheets wherever friction occurs on edges, turning bars, etc., or where two materials touch and subsequently separate again (separating events), for example at rollers. They may result in production disturbances, dust accumulations, the influencing of measuring and control instruments and even in explosions. Great static charges (more than 5000 V) can ignite gas/air mixtures of vaporizing solvents, and small static charges of less than 1000 V can optically impair or destroy the surface by the formation of streaks, dust marks and accumulations of dirt.
The size of the electrostatic charge depends on the electrical conductivity of the materials. Good conductors are, for example, metals, carbon black and graphite, which lose their charge immediately, while poor conductors, in particular untreated polymers, retain their charge over lengthy periods of time, namely seconds to minutes and even several days.
On plastics parts, generally their poor antistatic properties are undesirable. Electrostatically highly charged plastics parts present enormous difficulties during subsequent upgrading steps, such as for example coating, thermoforming or printing.
In order to overcome these difficulties, it is state of the art to perform chemical-physical surface modifications of plastics, in particular of films. A basic process, which only brings about changes on the surface of the plastic, consists in a treatment of the surface of the plastic by an electric corona discharge. For instance, according to German reference DE-A-3,247,795, a corona treatment of a plastic film web is performed by the film web being subjected on the upper side and/or underside to impingement by a corona, which produces different treatment intensities. For this purpose, the film web to be treated is passed over an electrically grounded roll and the electric charging is carried out by impinging one or both sides of the film web with an electric corona discharge, which is generated by applying a high-frequency alternating current of high voltage to an electrode arranged at a distance from the roll. The treatment is generally performed in air at atmospheric pressure.
The constantly increasing demands of the market for products with improved surface characteristics have also resulted in the development of processes using chemically reactive substances, which for example, break up certain chemical bounds in the surface and, as a result, change the surface characteristics of plastics.
A process for the chemical-physical surface treatment of plastics moldings is described in German reference DE-A-3,705,482, in which selected reaction mechanisms are initiated on the treated substrates by incorporation of liquids atomized into aerosols into an alternating current corona discharge. In the apparatus, the substrate to be treated is passed through a discharge gap, which is formed by a roll at ground potential with a dielectric covering as counter electrode, and with electrodes under high voltage, and into which an aerosol is blown from a separate atomizer device by means of a carrier gas.
It is also state of the art to expose sheet-like substrates to a corona discharge on both surfaces, in order to achieve a treatment on both sides. For instance German reference DE-B-1,779,400 describes an electrode arrangement in which the film web to be treated wraps around two roll electrodes in the shape of an "S". However, this apparatus is not suitable for allowing an aerosol to act on both surfaces of the film during the corona discharge. The direct contact of the first-treated substrate surface, possibly still moist from the aerosol, with the following corona roll may result in damage to the aerosol layer applied, and there is also the risk of electrical breakdowns occurring on the substrate, and possibly also the electrode roll being destroyed. Although the apparatus known from British reference GB-A-722,875 for the two-sided treatment of plastic films by corona discharge is suitable for the direct simultaneous corona treatment of a web-shaped substrate, it rules out the selective feeding of an aerosol into the discharge gap on account of its compact, closed construction.